TechnicalArticle Driving time-multiplexed LED arrays at high current: a new approach Joel Gehlin Technical Article Driving time-multiplexed LED arrays at high current: a new approach Joel Gehlin System designers have adopted time-multiplexed architectures for large-scale LED matrices in recent years in order to achieve a large reduction in the number of current sinks/sources required. for many large display applications. Basic operation of time-multiplexed matrices Time-multiplexing is a technique for driving LEDs in a matrix without requiring a source for every LED. LED D5 is controlled in the same way via Source. This article describes a solution to the problem using a device type that is far from the most obvious choice: LED driver ICs for TV backlighting. Close examination of the datasheets. The rapid growth in the market for LED TVs has spawned a new generation of sophisticated and highly efficient driver ICs that provide a high current capability. This is because of the refresh rate applied by the LED driver in order to spread the current evenly throughout the LED matrix if more than one LED is on at the same time. Figure 1 shows the operation of a time-multiplexing scheme. Page 2 / 8 . As a result. Sink. Source. (The meaning of ‘refresh rate’ is explained below. however. The article will explore the operation of time-multiplexing schemes and show the extent to which the capabilities of backlighting driver ICs match the requirements of large lighting and signage systems. is more difficult to implement when a high current is required at the LEDs.2 and Sink. a real 150mA supply would be adequate – but the refresh rate applied in time-multiplexed architectures means that the effective peak current at the LED is often one-half or one-third of the chip’s nominal peak.1 needs to be supplied with a voltage higher than the maximum forward voltage (VF) of the LED.1 needs to be connected to a resistor or other type of current sink to draw the current through the LED. This architecture reduces the size and cost of the electronics circuitry in end products with large LED arrays. however. To control the LED D1. only a few can deliver even as much as 150mA.2. conventional LED driver ICs tend to be able to drive a high number of LEDs in a matrix configuration. In fact. such as smart commercial lighting and RGB signage.) Interestingly. low cost and small size using conventional LED driver ICs. This kind of design. reveals the problem: the constant current at each sink/source in matrix configurations is typically in the range 10-40mA. designers have found it hard to combine high current output (producing high brightness) with high efficiency. D2 and D4 will be turned ON as well. Let’s say that a given matrix refresh rate for a given set of lit LEDs produces an effective 50% duty cycle applied to the LEDs: at a current set to 100mA via the current sink. Here.1/Sink.1 and Sink. There might appear to be an obvious way to combat this effect: double the current at Sink. To overcome the problem.2 ON continuously. But what if D1 and D5 need to be ON at the same time? If Sink. Sink. There is. An example of a matrix control scheme is shown in Figure 2.1/Source. Provided the flickering of LEDs D1 and D5 is at a frequency of 50Hz or higher. D1.2. the light will appear to the human eye to be continuously ON. Page 3 / 8 . the concept of time-multiplexing must be used. D5 and D9 are being lit with a current of 100mA through each LED.Technical Article Fig. so easy. a current output of 200mA is beyond the capability of the conventional LED driver ICs on the market today. Source. Unfortunately. Instead of turning Source.1 and Source.1 and Sink.2 and Source. This time-multiplexing technique using an effective refresh rate faster than 50Hz thus permits D1 and D5 to be lit without lighting D2 or D4.2.2/Sink.2 are all ON. of course.2 to 200mA to provide a constant current of 100mA though the LEDs. the driver multiplexes between Source.1/Sink.1: the operation of an LED time-multiplexing scheme So far.1. the effective constant current through each LED is 50mA. a drawback: the time-multiplexing with the associated refresh rate reduces the total LED current passing through the LEDs. Time-multiplexing control scheme The refresh rate describes the number of times per second that the current through each lit LED in the matrix is reset. Time-multiplexing also enables the creation of animations. a refresh rate of 200Hz for the entire matrix means that each LED will be switched at around 67Hz.Technical Article Fig.2: time-multiplexing control scheme Provided a multiplexing scheme is looped fast enough – between 200 and 1. either via external MOSFETs or via FETs embedded in the driver chip. A new approach to driving time-multiplexed LED architectures at high current TV backlighting designs have almost universally replaced incandescent light sources (CCFL tubes) with LEDs. To realize the picture.000 times per second. high-precision LED controller with built-in PWM generators for driving external FETs.) While the AS3693 family was specially designed to meet the precise currentPage 4 / 8 . Because of the requirement for high brightness in TVs. depending on the number of LEDs to be lit simultaneously – the LEDs will appear to the human eye to be continuously ON. a 16-channel. In Figure 2. (A sister part. which corresponds to a duty cycle at each LED of 33%. A huge market segment. the AS3693A. features integrated MOSFETs. the controller must scan through each array one byte at a time. This means that each sink needs to handle at least 300mA in order to produce the constant current equivalent of 100mA at each LED. The animation may be created in software code with a pre-defined series of bitmap images: these are usually arrays of n-bytes. these ICs must be able to control LEDs at high currents. An example of such a device is the AS3693B from ams. displaying one column after another. in which each bit represents one LED in the LED matrix. TV backlighting has induced a surge in the number of specialized backlighting LED driver ICs. Particularly useful is the ability to program output currents. such as: • Multi-pixel advertising boards • Traffic signals • Backlit signage • General illumination • Accent lighting with RGB color-changing capability Demonstration: LED ‘EXIT’ sign Safety EXIT signs powered by LEDs are up to 90% more efficient than traditional incandescent signs. LED EXIT signs also offer savings in maintenance and repair. Operating 24 hours a day. the devices may also be used to source/sink and control LEDs in other applications. which is also available in the AS382x family. which has no PWM generator. This second. conventional design will occupy a much larger PCB area and incur a much higher bill-of-materials cost. The options include: • Independent digital current control for each channel with a PWM generator • Linear current control with an 8-bit DAC • Linear current control with an external analogue voltage Together with the ability to control external MOSFETs. since their re-lamping cycle is typically a very long period of around 10 years. Figure 3 shows how a single AS3693B – a 16-channel driver IC with an integrated PWM generator – can be configured to control a time-multiplexed matrix of 60 white LEDs. In addition. (An almost identical circuit could also control 3 x 20 RGB LEDs. the cost and energy savings to be made by switching to LEDs are thus very considerable. which would require four 16-channel driver ICs of the conventional type. LED signs can offer better optical performance. Page 5 / 8 . the AS3693B gives the designer the freedom to set an appropriate maximum current and adapt the output to the needs of a variety of applications.Technical Article control requirements of TV manufacturers.) The use of such a device in a time-multiplexed architecture offers the system designer considerable bill-of-materials and space savings compared to a conventional design. Selection of an appropriate NMOS device will provide for high efficiency and high brightness. the application can be dimmed. DAC CURR_sense7 GND GND 33 CURR_sense13 PWM Gate5 Gate12 RFB12 AS3693B CURR_sense5 RGS RFB16 PMOS 36 RGS 35 NPN RSET 34 GND PMOS 45 46 RGS NPN RSET 47 GND PMOS 50 RGS 48 NPN RSET 49 GND PMOS 53 RGS 51 NPN RSET 52 GND PMOS 56 RGS 54 NPN RSET 55 GND PMOS 57 59 NPN RSET 58 GND D1 D2 D3 D4 D5 D6 GND D7 D8 D9 D10 D11 D12 GND D13 D14 D15 D16 D17 D18 D19 D20 D21 D22 D23 D24 D25 D26 D27 D28 D29 D30 D31 D32 D33 D34 D35 D36 D37 D38 D39 D40 D41 D42 D43 D44 D45 D46 D47 D48 D49 D50 D51 D52 D53 D54 D55 D56 D57 D58 D59 D60 60 NMOS 62 RSET 61 63 NMOS 1 RSET 64 V2_5 Fault GND RFB4 PWM SDO 23 CURR_sense12 PWM Gate4 44 RSET RFB11 CURR_sense4 43 24 NMOS 22 Gate11 RFB3 SDA GND PWM SCL 20 CURR_sense11 PWM Gate3 CS 19 RFB10 CURR_sense3 42 21 NMOS RSET Gate10 RFB2 Hsync GND PWM 41 17 CURR_sense10 PWM Gate2 Vsync RSET 16 RFB9 CURR_sense2 40 18 NMOS Gate9 RFB1 39 GND PWM 38 15 FBR 9 RSET 14 V2_5 Vreg 13 NMOS AS3693B CURR_sense9 PWM Gate1 ADDR1 GND CURR_sense1 GNDsns 2 8 3 GND 10 GND 12 4 NMOS RSET Vled C4 4.Technical Article The constant current through each LED is provided by NMOS transistors. Vpow FB Vreg V2_5 C3 C2 100nF 25 NMOS RSET 27 26 GND 28 NMOS RSET 30 29 GND 31 NMOS RSET 32 6 37 7 5 FBG FBB REF(Ext) PWM Gate13 RFB5 RFB13 CURR_sense6 CURR_sense14 PWM Gate6 PWM RFB6 PWM Gate15 SMPS feedback RFB7 CURR_sense8 RFB15 CURR_sense16 86 byte registers PWM Gate8 PWM Gate16 SPI / I2C Interface RFB8 65 CURR_sense15 Fault detectors PWM Gate7 Gate14 RFB14 Reference. During daytime. The maximum current can be limited by the RSET resistor connected to the source of the external MOSFET in each current sink.2uF 100nF GND ADDR2 GND 11 C1 Rpullup Fault GNDGND RADDR1 RADDR2 Rpullup Vreg Rpullup GNDGND Rpullup HSYNC VSYNC RVSYNC SDO RHSYNC SDA SCL GNDGND CS Fig. Page 6 / 8 .7uF 2. 3: 60 time-multiplexed white LEDs controlled in a time-multiplexing architecture by a single AS3693B from ams To reduce power consumption even more the AS3693’s current settings can also adapt to the ambient light. once the light becomes too dark. the system can boost the brightness to produce higher visibility and contrast. The operation of such a circuit is shown in Figure 4.Technical Article Improving efficiency through intelligent DC-DC conversion The LEDs may be connected directly to. for example. It senses its output voltage with a resistive voltage divider. Page 7 / 8 . 4: AS3693B feedback block for adjustable voltage control The output of pins FBR. Each PWM generator in the AS3693B can be independently selected to use any of the three feedback pins. This voltage divider can be modified to set the output voltage between a minimum output voltage (VMIN) and a maximum output voltage (VMAX) which is the basis of the device’s dynamic feedback control. Fig. the LEDs can equally be powered from an external DC-DC converter that can dynamically change its output voltage to match the needs of the LEDs’ VF. The AS1341 is an efficient stepdown converter with adjustable output voltages ranging from 1. but this will reduce the design’s efficiency as power is dissipated as waste heat in the NMOS transistors. The AS3693B offers three different paths for feedback. FBG and FBB of the AS3693B can be used to control any external power supply.25V up to the input voltage (20V maximum). and each LED can be assigned to a specific LED supply. To optimize efficiency for battery-powered equipment. a 12V power supply. Conclusion By implementing a time-multiplexing scheme with an LED backlight driver IC and combining this architecture with an adaptive power supply.com Page 8 / 8 . For further information ams AG Tel: +43 (0) 3136 500 [email protected] AS382xfamilies. For more information about the ams portfolio of LED drivers.Technical Article Alternatively.com www.ams. the designer of a large LED array can gain impressive savings in board area. bill-of-materials cost and power consumption. the default register setting will function as a general feedback to the power supply.ams. by tying all the feedback pins together as a single feedback from AS3693B. including the AS369x. Moreover. this design involves the use of readily available standard parts that are well supported by relevant documentation and specifications.com/eng/Products/Lighting-Management/Large-LCD-Panel-BacklightingLED-Drivers. visit www.
Report "ATA1235 High Current Time Multiplexed LED Driving"